The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro vs Wegovy — Which Is Right for You?
If you're exploring prescription weight loss options in the UK, you've likely seen both Mounjaro and Wegovy mentioned. Both are prescription-only GLP-1 medications approved for weight management, but they work differently and have different costs. This guide compares the two side-by-side so you can understand the key differences and have an informed conversation with your GP or a licensed prescriber. Individual results vary, and eligibility depends on your personal health circumstances.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
How Mounjaro and Wegovy Work — Clinical Trial Comparison
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways in the body. This dual mechanism may explain why clinical trial data showed stronger average weight loss results. In the SURMOUNT-1 trial, participants taking 15 mg Mounjaro lost an average of 20.9% of their body weight over 72 weeks.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, working through a single hormone pathway. In the STEP-1 trial, participants on 2.4 mg Wegovy lost an average of 14.9% of their body weight over 68 weeks. Both medications are once-weekly injections and are used alongside a reduced-calorie diet and increased physical activity.
- Mounjaro doses: 2.5 mg to 15 mg weekly
- Wegovy doses: 0.25 mg to 2.4 mg weekly
- Both require 4-week periods at each dose before escalation
- Individual results vary significantly
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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Cost, Eligibility, and How to Access in the UK
Both Mounjaro and Wegovy are prescription-only medicines requiring a clinical assessment before dispensing. On the NHS, access is through specialist tier 3 weight management services—availability varies significantly by region and waiting lists can be lengthy. Privately, both are available through GPhC-registered online pharmacies following an online clinical assessment; costs typically range from £129–£320 per month depending on the dose.
Eligibility for both is the same: BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Your prescriber will assess which medication is most suitable based on your individual health profile, medical history, and any other medications you take. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to check your starting point.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is Mounjaro cheaper than Wegovy in the UK?
Costs vary by provider and dose, but prices are generally comparable—typically £129–£320 per month for either medication privately. NHS access is free but availability depends on your region and specialist service capacity. Your prescriber cannot recommend a medication based on cost alone; clinical suitability is the priority.
Which causes fewer side effects—Mounjaro or Wegovy?
Both share common side effects like nausea, diarrhoea, and constipation, especially during dose escalation. Individual tolerance varies. Some people experience fewer side effects with one than the other, but this cannot be predicted in advance. Your GP or prescriber will monitor you and can adjust your approach if needed.
Can I switch from Mounjaro to Wegovy or vice versa?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart—approximate equivalence exists (e.g., 15 mg Mounjaro ≈ 2.4 mg Wegovy), but any switch requires medical oversight and typically involves a period off medication.
How long does it take to see weight loss results with these injections?
Weight loss typically begins within the first 2–4 weeks, but most significant results appear after 8–12 weeks of consistent use at therapeutic doses. Clinical trials measured results over 68–72 weeks. Faster results are not always sustainable—steady, gradual loss is the goal.
What if I don't lose weight on Mounjaro or Wegovy?
Not everyone responds equally to these medications. If weight loss plateaus or doesn't occur after 12 weeks at the maximum tolerated dose, your prescriber may discuss alternative approaches. Adherence to diet and activity changes is essential for these medications to work effectively.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.